1. Field of the Invention
The present invention relates generally to semiconductor memory devices and more particularly to a circuit for generating a substrate voltage and a pumped-up voltage with a single oscillator.
2. Description of the Related Art
In general, an operating voltage of a semiconductor memory device tends to decrease as the density of semiconductor memory devices on a chip increase. For instance, a 16Mbit dynamic RAM uses an operating voltage of 4V whereas a 64Mbit dynamic RAM uses a 3.3V operating voltage. The operating voltage is derived from an external supply voltage of 5V. Along with the trends toward lowering operating voltages, continuous research has been made into reducing the power consumption of semiconductor memory devices. To that end, a circuit for generating a constant voltage is generally included in semiconductor memory devices. For instance, in a semiconductor memory device such as a dynamic RAM or pseudo static RAM comprised of one storage capacitor and one access transistor, a substrate voltage generator is necessarily used to retain data stored in the storage capacitor and a noise margin of the memory device. In addition, a boosting circuit or a voltage pumping circuit is also used for generating an increased voltage higher than the external supply voltage in order to compensate for the voltage loss by the access transistor, caused by the driving operation of a word line coupled to a memory cell to be accessed.
FIG. 1 illustrates a conventional substrate voltage generator. An oscillator 1 generates pulses .phi.OSC upon power-on of a semiconductor memory device. A substrate voltage driver 2 receives the pulses .phi.OSC to generate complementary driving signals. A substrate voltage pumping circuit 3 generates a substrate voltage V.sub.BB in response to the driving signals from the substrate voltage pumping circuit 2. A substrate voltage detector 4 detects the level of the substrate voltage output of the substrate voltage pumping circuit 3 to control operation of the oscillator 1 according to the detection. The substrate voltage detector 4 controls the operation of the oscillator 1 through a transistor 6 and inverter 5. In operation, the oscillator 1 generates the pulses .phi.OSC having a period of 60-100 ns upon power-on of the semiconductor memory device. Then, the substrate voltage driver 2 receives the pulses .phi.OSC and generates complementary square wave driving signals. The substrate voltage pumping circuit 3 pumps up the substrate voltage V.sub.BB in response to the driving signals until the substrate voltage reaches a predetermined level. Meanwhile, if the substrate voltage V.sub.BB reaches the predetermined voltage, then the detector 4 causes the oscillator 1 to stop generating the pulses .phi.OSC in response to the detection result. On the contrary, however, if the substrate voltage V.sub.BB is below the predetermined voltage level, the detector 4 allows the oscillator 1 to generate the pulses .phi.OSC so as to pump up the substrate voltage V.sub.BB.
FIG. 2 shows a conventional voltage pumping circuit. The structure and operation of the voltage pumping circuit are similar to those of the substrate voltage generator of FIG. 1. However, a voltage pumping circuit 13 replaces the substrate voltage pumping circuit 3 and generates a pumped-up voltage instead of the substrate voltage. A pumped up voltage detector 14 replaces the substrate voltage detector 4 also. The pumped-up voltage is generally used as a supply voltage of the peripheral circuits such as a data output buffer and a word line driver.
The substrate voltage generator shown in FIG. 1 and the voltage pumping circuit illustrated in FIG. 2 are both necessarily included in a semiconductor memory device. The substrate voltage pumping circuit 3 of FIG. 1 is comprised of a number of MOS capacitors for pumping up the substrate voltage negatively. A voltage pumping circuit 13 of FIG. 2 is also comprised of a number of MOS capacitors for pumping up the pumped-up voltage higher than the external supply voltage. Conventionally, the substrate voltage generator of FIG. 1 and the voltage pumping circuit of FIG. 2 have separate oscillators 1 and 11.
A typically known oscillator which may be used for the oscillators 1 and 11 is described in detail in FIG. 3. The oscillator shown in FIG. 3 is described to emphasize the amount of current a typical oscillator drains. Therefore, a complete functional description is omitted. The operation of the oscillator in FIG. 3 should be apparent to those skilled in the art.
Such an oscillator consumes a large amount of current, particularly during a stand-by mode of the semiconductor memory device. For example, if it is assumed that the operating voltage and the total current consumption of the semiconductor memory device during the stand-by state are 3.3V and 50 .mu.A respectively, the current consumption of the oscillator is 20 .mu.A which becomes 2/5 of the total current consumption of the semiconductor memory device. Such stand-by current by the oscillator prevents reduction of the current consumption of the semiconductor memory device. Accordingly, if two separate oscillators are used for a semiconductor memory device, the stand-by current of the oscillators becomes 4/5 of the total current consumptions of the semiconductor memory device thereby resulting in an increase of the current consumption.